Meta·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Meta data scientist round with a string template expansion problem. Interesting problem but the edge cases around cycles and missing keys are where things get tricky, and I don't think I handled them as cleanly as I could have.

Questions Asked (1)

You have a dictionary mapping string keys to template values, where templates can contain placeholders referencing other keys. Given an input string with placeholders, return the fully expanded string. How do you handle nested references, missing keys, and cycles like A referencing B which references A?

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The base case is easy enough, just do a find-and-replace pass.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints (e.g., can placeholders appear anywhere, are values always strings, what should happen on missing keys or cycles). Then propose a recursive expansion with memoization and cycle detection, discussing trade-offs between eager vs lazy expansion and error handling strategies.

Pro tip: Mention that you would treat this as a graph traversal problem and use DFS with a 'visiting' set to detect cycles, and that you'd cache expanded results to avoid redundant work—this shows you think about both correctness and efficiency.

1. Clarify requirements and edge cases

Ask about placeholder syntax, whether values can be non-strings, expected behavior for missing keys (error, leave as-is, empty string), and whether cycles should raise an error or be handled gracefully.

2. Model as a dependency graph

Treat each key as a node and each placeholder reference as a directed edge. This helps reason about cycles and topological order for expansion.

3. Design recursive expansion with memoization and cycle detection

Use DFS to expand each key, maintaining a 'visiting' set to detect cycles and a memo dictionary to cache fully expanded values. On cycle, either raise an error or return a placeholder.

4. Handle missing keys and substitution

Decide on a policy for missing keys (e.g., raise KeyError, leave placeholder, or replace with empty string) and implement string substitution accordingly, ensuring nested expansions are resolved.

5. Analyze complexity and discuss trade-offs

Explain time complexity O(N + E) where N is number of keys and E is number of references, and space O(N) for memoization. Discuss trade-offs between eager vs lazy expansion and error handling strategies.

Key Points to Mention

Cycle detection using a 'visiting' set or coloring (white/gray/black) during DFS.
Memoization to avoid re-expanding the same key multiple times, improving efficiency.
Handling missing keys: options include raising an exception, leaving the placeholder, or substituting with a default value.
Recursive vs iterative expansion: recursion is simpler but may hit stack limits; iterative with explicit stack is more robust.
String substitution method: regex or manual parsing to find and replace placeholders.
Complexity analysis: O(N + E) time and O(N) space, where N is number of keys and E is number of references.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.